Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to nonvolatile semiconductor memory devices and related methods of operation.
Semiconductor memory devices can be roughly divided into two categories including volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM), and examples of nonvolatile memory devices include electrically erasable programmable read only memory (EEPROM), ferroelectric random access memory (FRAM), phase-change random access memory (PRAM), magnetoresistive random access memory (MRAM), and flash memory.
Flash memory is a form of nonvolatile memory known for having relatively high programming speed, low power consumption, and high storage capacity. Consequently, flash memory has been adopted as a storage medium in a wide variety of electronic devices.
To increase the integration density and storage capacity of flash memory devices, researchers have developed flash memory cells capable of storing more than one bit of data. A flash memory cell storing one bit of data is called a single level cell (SLC) and a flash memory cell storing two or more bits per memory cell is called a multi-level cell (MLC). An SLC typically has two states corresponding to two different threshold voltage distributions. These states include an erased state and a programmed state. A multi-level cell typically has additional states corresponding to additional threshold voltage distributions.
In a flash memory having MLCs (hereinafter, an MLC flash memory), different threshold voltage distributions should be separated by sufficient margins to allow the corresponding states to be distinguished from each other during read operations.